Manufacture of transparent paper



Patented a. 22, 1935 UNITED STATES MANUFACTURE OF TRANSPARENT PAPER William Michael Driesen, Clifton, N. J., assignor to S. D. Warren Company, Boston, Man, a corporation of M2 wsachusetts No Drawing. Application October 5, 1928, Serial No. 310,671

10 Claims.

My invention relates to the manufacture of transparent paper, and has for its object the production of paper of such heightened transparency that it may be successfully used, for instance,

5 as a wrapper for printed or decorated packages and exhibit their printing or decoration without blurring the opticalimage or seriously obscuring or depressing colors. For such purposes the transparentized paper which exemplifies the sheet material product must retain as predominant characteristics substantially the same thinness, pliability and manipulability as manifested by the fibrous base-sheet which is treated according to my herein described invention. Many of the cellulosic fibers of which paper is made are of a substance inherently transparent, but their irregularities of surface and small diameter, and matted arrangement in the paper web, cause such reflections and dispersions of light as to produce opacity of the paper as a whole to a substantial degree. The inclusion, between the fibers, of air which has an index of refraction substantially different from that of the fiber substance, contributes to the opacity of the paper sheet.

Paper having a practicable transparency in limited range, like the glassine paper used for window-envelopes has been produced by mechanically condensing a thin sulflte fiber paper, as by calendering; this treatment to some extent eliminates the factors productive of opacity, but nevertheless leaves the paper effective only to disclose the outlines of objects such as printed letters when the paper which bears them lies close to or in contact with, the envelope window paper. For many commercial purposes this inferior degree of transparency is inadequate, and thin flexible cellulosic films, prepared in sheets, have been quite largely employed where high transparency is demanded, even in spite'of their relatively high cost.

It is well known that if a mass of irregular solid bodies, of inherently transparent substance, opaque as an aggregate by reason of diffraction, dispersive reflection, and inclusion between the bodies of a fluid, and therefore displaceable, substance of refractive index markedly different from that of the material of said bodies, be immersed in and intimately wetted by, a transparent liquid having the same or approximately the same refractive index as the bodies, the composite mass becomes transparent, the solid fragments disappearing from view. Exposed reflective and diffractive surfaces and the interstitially included air have been eliminated, the composite body has become virtually optically continuous; if the 5 boundary surfaces of the liquid be regular (as in a glass jar) the transparency of the composite mass will be nearily if not quite as perfect as would be a volumetrically identical mass of either component. 10

Paper is usually made of vegetable fibers; the material of which these fibers are composed is predominantly cellulose; this material has the same or very nearly the same, refractive index as amorphous celluloses, or cellulose derivatives, 15 such as cellulose nitrate or acetate, etc. Many amorphous celluloses have the capacity to form transparent, tenuous and flexible films on evaporation of a solution in any one of their volatile solvents. A solution of such a cellulose derivative 20 in an appropriate volatile solvent is-especially when dilutecapable of permeating a paper web, and intimately wetting the component fibers; a suitable paper (by which is meant a paper substantially free from filling materials, and constituted as simply as may be of the vegetable cellulosic fibers alone) while a dilute cellulosederivative solution with which it has been impregnated, is still fluid, is more clearly transparent than before impregnation, for the reason 30 that it is, as a whole, rendered more perfectly optically continuous from side to side.

But, at or after the stage where the evaporation of the volatile solvent reverses the phase of the dispersed cellulose derivative, and when the residual amorphous cellulose assumes or approaches a substantially solid condition, the previously enhanced transparency of the sheet gives place to a degree of opacity higher than that of the original unsaturated sheet. This phenomenon can beattributed only to destruction or degradation of optical continuity, since the cellulosic film itself is capable of assuming clear transparency and does not differ materially from fiber-substance in refractive index. As a'physical condi- 45 tion of virtual optical continuity is the elimination of laminae of interruption between the fibersurfaces and the impregnant solution, due to the intimate wetting of the one by the other, the marked deterioration in transparency observed as the cellulose solvent is dissipated is doubtless due to retraction of the cellulose from the fiber surfaces and the production of laminae of interruption which destroy or degrade optical continuity; for, this deterioration in transparency supervenes even when conditions are such as to preclude cloudiness or opacity in the cellulose film material itself.

A means, therefore, by which a paper web may be so treated as to assume and retain a high degree of transparency, according to this invention, is the inclusion of a cellulosic fiber paper web in, or its impregnation by, a substantially solid, flexible film-matrix, of substance having substantially the same index of refraction as the paper-fiber substance, in intimate, virtually optically uninterrupted contact with the fiber-surfaces, and as a whole, substantially coincident in thickness with the paper-web itself, so that the two surfaces of the film are practically even and free from irregularly reflecting or diifracting contours, and the sheet material as a whole possesses in substantially unimpaired degree the flexibility and manipulability of the paper base. Such a film-matrix will necessarily be freely fluid, as by solution of the film-forming material in a volatile solvent, for purposes of impregnating the sheet of fibers and intimately wetting the surfaces of the fibers themselves, and must also be so constituted or qualified that on assuming a solid consistency, the intimate contacts between film material and fibers, subsisting while the former remained fluid, shall not be relaxed. The intimacy of liquid-to-fiber surface relationship must be persistent, though the film material becomes, to all practical intents, a solid body.

In that aspect of it which involves the use of film-forming cellulose derivatives, my invention is characterized therefore by the permanent maintenance of solution-relationship, in the broad physical sense, between the cellulose derivative and a solvent thereof which is,-(as contrasted with the low boiling point volatile solvent or solvents used as a vehicle for initial impregnation of a paper web with the film-matrix material) normally non-volatile at ordinary atmospheric temperatures, and consequently a permanent component of the film-substance. This ingredient may therefore be termed a stable solvent of the cellulose derivative. The relative proportions of the stable solvent and the cellulose'derivative will preferably be such that when the cellulose derivative has set to the virtually solid condition in film form, it constitutes the continuous phase of the mutual dispersion with the stable solvent, the composite body represent-'- ing the phenomenon of "solid solution".

The physical conditions or characteristics of the stable solvent are: dispersive miscibility with, or solubility in, the volatile solution of the cellulose derivative; normal non-volatility at ordinary atmospheric temperatures; and index of refraction near enough to that of cellulose to preclude disturbance of the substantial optical identity between the paper fiber cellulose and the filmmatrix. The first characteristic is satisfied if the stable solvent is soluble or dispersible in a solution of cellulose in a volatile solvent, even though the stable solvent be not directly miscible .with,

.or solvent of, the cellulose derivative itself; it

sufiices if the stable solvent, having dispersed in the solution, remains dispersed in the residual solid after the volatile solvent has evaporated.

Specific examples of formulae and procedure which I have demonstrated to be effective to produce highly transparent paper, are as follows:

For the better performance of the related functions above indicated, take, as the paper base, or fibrous framework for the improved transparent 5 sheet, a web of partially hydrated vegetable fiber, in particular and by preference a Mitscherlich process sulfite fiber stock, worked a long time in the beater, and, after formation of the paper, re-moistened with water and condensed by heavy l0 calendering on steam heated rolls. I believe that the condition of such fiber, due to a considerable partial hydration, is highly conducive to intimate surface association of fibers and an impregnant cellulosic solution, and that the con- 15 densation of the paper web further assists, not only in reducing the thickness of'the web as such, but also by flattening the fibers and increasing their individual specific surfaces.

Having made paper of desired thickness and 20 quality by such means, dry it, and impregnate it, by spraying or brushing with, or (preferably) by immersion in, a solution constituted as follows (percentages are by weight) Per cent 25 Nitrocellulose (ester soluble) 5.7 Denatured alcohol 2.4 Ethyl acetate 70.1 Amyl acetate 5.5

Butyl acetate 9.3 30 Ethyl lactate 3.7 Diamyl phthalate 3.3

The cellulose nitrate which is recommended for this and the alternative formula to be stated is that known as two second" nitrate, according to the viscosity standards employed by manufacturers of high explosives. Nitrocelluloses of 4 other viscosities may be employed, according to the consistency and quality of the film-material desired in the completed transparent paper product. .The paper may with some advantage be impregnated while in an exhausted chamber, and the volatile solvents eliminated from the impregnant under similar pressure conditions. In the foregoing formula, diamyl phthalate represents what is hereinabove designated as the stable solvent. It is liquid and practically non-volatile at 50 ordinary temperatures, boils at about 300 C., is soluble in the cellulose solution and remains in mutual dispersion or solution with the nitrocellulose after the volatile solvents have been evaporated and the cellulose set in the form of a flexi- 55 ble film, encasing the paper web and the fibers thereof. The degree of intimate, optically uninterrupted contact between the film substance and the fibers, which was established when the freely fiuid composition impregnated the paper web and V wetted the fiber surfaces, persists in the finished 60 Another formula for impregnating solution which has yielded good results, is as follows:

The treatment of the paper web with this solution may be the same as prescribed for the first specified solution. The stable solvent is here represented by the anhydrous lanolin. While treatment of the paper with this solution directly and without previous preparation produces a highly transparent product, I believe that the results can be improved in degree by giving the paper, which has been well dried, a preliminary bath or impregnation witha dilute solution of anhydrous lanolin, in a volatile solvent such as ethyl acetate, evaporating the volatile solvent, and thereafter impregnating the thus prepared paper with the full formula above given. The exact degree of dilution of the preparatory solution of stable solvent is not important; a very fluid solution is recommended, because the purpose of such pretreatment is to carry into every penetrable part of the paper fiber a coating of stable solvent. This preliminary coating apparently has the effect of promoting or facilitating similarly complete penetration of the fiuid constituted by the fullformula. Another reason for using a very dilute solution of stable solvent in preliminary impregnation is, that the deposit of stable solvent should not be so abundant as to afiect substantially the ascertained optimum percentage of the said solvent in the full formula.

Both the foregoing formula: for impregnating solution were empirically determined by using final transparency of the paper product as the standard of reference. The range of specific variation from such specified formulae, consistent with obtainment of enhanced transparency with any given paper base, is obviously large. Even within the limits of nitro celluloses there is a very considerable margin for choice; the viscosities according to the seconds standard vary from half-second nitro cellulose to three thousand seconds nitro cellulose, according to their nitration variations, yet all are capable of forming virtually solid films from volatile solutions. Another cellulose derivative class is the acetates. The adaptability of representatives of the several classes of film-forming cellulose derivatives (the catalogue here given is not intended to be exhaustive) to form tenuous films from volatile solutions is in general well enough known to enable a well advised selection to be made among them for purposes of the paper treatment for enhanced transparency. Stable solvents, equivalent (as judged by the test of enhanced transparency of treated paper) to those specified in (B. P. 350 C.) triphenyl phosphate (B. J. 245 C.) butyl stearate (B. P. 330 C.) iso amyl phthalate (B. P. 340 C.). (These boiling points are approximate, for normal atmospheric pressure.) Diamyl phthalate (B. P. 340 C.) is functionally 5 effective to produce stable enhanced transparency when used alone, instead of in mixture, as the stable solvent for a cellulose derivative film.

Mixtures of stable solvents have been found functionally effective in cellulosic solutions, to 10 produce permanent enhanced transparency in paper. But other characteristics than transparency are acquired by the paper after treatment with some solutions containing some of these high boiling, non-volatile; stable solvents, 15 such as tackiness or stickiness, and, unless such qualities are either desired, or neglible, in the light of the utilities to be served by the paper product, the employment of such stable solvents should be avoided, or means contrived to avoid 20 such properties in the finished paper product.

It will be observed that the above enumerated stable solvents'typically and in common include high boiling liquid esters, each comprising a basic organic radical, and are fully solu- 25 ble in volatile solvents of nitro-cellulose. The stable solvents are soluble in the volatile solvents of the cellulosic derivative film material and therefore blend with them.

In the practice of paper treatment above de- 30 scribed, care should be taken to eliminate water from the materials, because its presence in filmforming solutions is liable to cloud the deposited film.

I claim:

1. Sheet material, comprising a web of paper constructed substantially exclusively of fibers of transparent substance, and a film-matrix of celluiosic derivative film-material with anhydrous lanolin dispersed therein, said film-matrix having 40 substantially the same index of refraction as the fiber substance, enclosing the fibers in direct optically uninterrupted contact with their surfaces.

2. Sheet material, comprising a web of paper constructed substantially exclusively of partially hydrated sulfite fibers, and a film-matrix of cellulosic derivative film-material with anhydrous lanolin dispersed therein, said film-matrix hav- 5o ing substantially thesame index of refraction as the fiber substance, enclosing the fibers in direct optically uninterrupted contact with their surfaces.

3. Sheet material, comprising a web of paper 55 constructed substantially exclusively of partially hydrated sulfite fibers, and a film-matrix of nitrocellulose with anhydrous lanolin dispersed therein, said film-matrix having substantially the same index of refraction as the fiber substance, enclos- 60 mg the fibers in direct optically uninterrupted contact with their surfaces.

4. Method of making transparent sheet material, comprising saturation of a paper web composed substantially entirely of partially hydrated 65 sulfite fibers with a cellulose nitrate in volatile solvent containing also anhydrous lanolin as a stable solvent of the cellulose-nitrate, evaporating the volatile solvent and leaving the cellulosic film with the stable solvent dispersed therein en- 70 casing the paper fibers in direct intimate contact with the fiber surfaces.

5. Method of making transparent sheet material, comprising saturation of a paper web composed substantially entirely of partially hydrated 75 sulfite fibers with a cellulose nitrate in volatile solvent containing ethyl lactate and also anhydrous lanolin as a stable solvent, evaporating the terial, comprising preliminary saturation of a paper web composed substantially entirely of partially hydrated sulfite fibers with a solution in a volatile solvent of anhydrous lanolin as a stable solvent of the film forming cellulose derivative to be subsequently applied, depositing said stable solvent upon the fibers by volatilization of the volatile solvent, thereafter saturating said web with a solution of film-forming cellulose derivative in a volatile solvent which is also a solvent of said stable solvent, evaporating the volatile solvent and leaving the film-material with stable solvent dispersed therein encasing the paper fibers in direct intimate contact with the fiber surfaces.

7. Method of making transparent sheet material, comprising preliminary saturation of a paper web composed substantially entirely of partially hydrated sulfite fibers with a dilute solution of anhydrous lanolin as a stable solvent of the film-forming cellulose derivative to be subsequently applied, depositing said stable solvent upon the fibers by volatilization of the volatile solvent, thereafter saturating said web with a solution of film-forming cellulose derivative in a volatile solvent which is also a solvent and contains in solution a supplemental quantity of stable solvent, evaporating the volatile solvent and leaving the film material with stable solvent dispersed therein encasing the paper fibers in direct inti-- mate contact with the fiber surfaces.

8. Method of making transparent sheet material, comprising saturation of a paper web com-'- posed substantially entirely of partially hydrated sulfite fiber with a cellulose nitrate in volatile solvent containing also anhydrous lanolin as a stable solvent of the cellulose nitrate, evaporating 5 the volatile solvent and leaving the cellulosic film in homogeneous condition with the stable solvent-dispersed therein encasing the paper fibers in direct intimate contact with the fiber surfaces.

9. Sheet material comprising a web of paper constructed substantially exclusively of fibers of transparent. substance and a substantially solid but freely flexible film-matrix of. nitrocellulose with a stable solvent dispersed therein, said film'- matrix having the same index of refraction as the fiber substance enclosing the fibers in direct optically uninterrupted contact with their surfaces, said stable solvent selected from a group consisting of anhydrous lanolin diamyl phthalate isoamyl phthalate, dibutyl phthalate, diethyl phthalate, butyl stearate, tricresyl phosphate, and triphenyl phosphate, said esters being characterized by high boiling points and solubility in volatile solvents of nitrocellulose.

10. Method of making transparent sheet ma- 8S terial comprising-saturation of a paper web constructed substantially exclusively of fibers of transparent substance with a cellulose nitrate in solution in a volatile solvent containing also'in solution a stable solvent, evaporating the volatile solvent and leaving the cellulosic film with stable solvent dispersed therein encasing the paper fibers in direct intimate contact with the fiber surfaces, said stable solvent being selected from a group 1 consisting of anhydrous lanolin diamyl phthalate, 86

isoamyl phthalate, dibutyl phthalate, diethyl phthalate, butyl stearate, tricresyl phosphate and triphenyl phosphate.

1 WILLIAM MICHAEL DRIESEN. 

